A Comparison: LeBlanc and Kassadin
In season 4, Both Kassadin and Leblanc became some of the premier mid lane assassins. With how overwhelmingly strong they both were, they were nerfed by riot in separate ways. Kassadin got a full rework that tried to remove his ability to teleport on to his targets, and kill them without them being able to retaliate what-so-ever (for the most part).
His silence was removed (rightly believed to be where most of his reliability as an assassin came from) and replaced with a shield that solely blocked magic damage. His ultimate was both buffed and nerfed. It got a lowered cooldown and damage that stacked when consecutively cast, but also ratios that scaled off of mana and a stacking mana cost. His Nether blade was changed from a dps mana stealing tool to one that helped offset Kassadin's early mana costs. His Force Pulse was largely kept the same (they just increased the necessary spells needed to be cast near him from 4 to 6). And his passive was changed from giving attack speed to giving magic resist and the ability to walk through minions. While still considered to be broken post re-work, after several buffs/nerfs, Kassadin is now no longer pick/ban. (2.3% pick .43% ban).
After Kassadin was changed, many started looking for which champion would replace his shoes as the premier mid-lane assassin. They eventually found their replacement in LeBlanc for many of the same reasons post re-work Kassasin was so strong in the first place.
She had what could basically be called a targeted silence and hyper-mobility that could keep the opponent guessing. However, she also had a slow that turned into a snare. an ult that allowed her to duplicate any of her basic skills, and a passive that gave her a brief moment of stealth and then a clone to help her confuse the opponent enough to either allow her to escape danger, or to finish off her opponent.
In order to balance Leblanc, the developers tried to target what made Kassadin so powerful, the silence. Her silence on procing her Sigil was removed along with her ability to double proc her sigil. Many thought that this would be the end of Leblanc as an assassin, and for a time they were right.
So why is Leblanc once again pick/ban status? That is something for the league community to discuss. Personally, I feel that it is because her kit gives her the ability to create long periods of tension for her opponent in the laning phase where she can either q them and just zone them, or quickly burst down 1-3/4 of their hp with a quick q-w combo then avoiding retaliation by returning back to her starting point. In the Late game, she has the potential to create tension for an entire team as her ability to very quickly burst then disappear allows her to pick off a team one by one while her team pushes or takes objectives.
That's it for this discussion, leave comments below and tell me what you think about this and why you think Leblanc is Pick/ban right now.
Have a cookie for doing so 
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now is how they can enter, exit and complete fights. Kassadin's damage rotation resets every ~9 seconds, and he has some free on-hit damage, so he can kill most targets given enough time and strategy, but his total damage is also the lowest out of any assassin, so he needs to go through multiple rotations to kill anyone. Leblanc, by contrast, has higher cooldowns on everything, including her ult, but has so much raw damage that she can kill any squishy in a single rotation. The fact that she can dive in, kill her target and retreat within the space of literally one or two seconds means she can be extremely difficult to handle properly, which is why she's among the most banned champions in the game. On top of this, it's pretty easy for her to snowball, since she can easily take out massive chunks out of her opponent's health in lane while remaining fairly safe at all times.
